This invention relates generally to opto-electronic semiconductor devices, and particularly, to a high band-gap quantum well heterostructure laser and a method of growing high band-gap semiconductor material.
An opto-electronic device serves to convert electric energy to light energy and vice-versa. It includes light emitting diodes (LEDs), laser emitters, photodetectors and photocells. In particular, LEDs are frequently used for displays and as indicators, and laser diodes, which may be regarded as a form of LED, are used as fiber-optic sources.
Various types of semiconductor LEDs are known. In most types of LEDs, a p-n junction semiconductor is employed. A potential difference is applied across the junction by means of a pair of electrodes in contact with the p-type and n-type regions. This causes electrons to be injected across the junction from the n-type region to the p-type region and causes holes to be injected across the junction from the p-type region to the n-type region. In the p-type region, the injected electrons recombine with the holes resulting in light emission; in the n-type region, the injected holes recombine with electrons resulting in light emission. The wavelength of the light emission depends on the energy generated by the recombination of electrons and holes which is determined by the band-gap of the p-n junction semiconductor material.
It is known in the art that the p-n junction may take on one of several forms. In the simplest form, a homojunction device is employed, where the p-type and the n-type regions are of the same band-gap energy. In improved LEDs, a single heterojunction device is employed, where the band-gap energy in the p-type region is different from that in the n-type region. This gives rise to the property that either electrons or holes, but not both, are injected across the junction. The injected electrons or holes then recombine to cause light emission in one region only. This region is commonly referred to as the active region. By concentrating the radiative recombinations in a smaller active region, a heterojunction device is more efficient than a homojunction device.
A device known as a double heterojunction LED further improves on the efficiency of single heterojunction LEDs. Typically the active region is sandwiched by a pair of wider band-gap layers, one being of p-type and the other of n-type. Two heterojunctions are thus formed from the triple layers. The higher band-gap of the additional layer helps to confine the injected electrons within the smaller band-gap active layer. This allows for a much thinner active layer which minimizes re-absorption and increases light emission efficiency. Furthermore, the pair of higher band-gap layers also acts as cladding layers which provide optical confinement to further enhance light emission efficiency.
In the case of laser emitters, a device known as quantum well heterostructure (QWH) is highly efficient. A QWH may be regarded as a double heterostructure where the thickness of the active layer is reduced to the order of carrier de Broglie wavelength. In this case, the motion of the carriers assumes a quantum effect and behaves like a two-dimensional gas localized within the plane of the active layer. The 2D quantization results in a series of discrete energy levels given by the bound state energies of a finite square well. The corresponding density of states acquires a step-like function. In contrast, the density of states for the non-quantum counterpart is described by a parabolic function and diminishes to zero as the band edge is approached. QWH are advantageous in that they have higher emission efficiency, faster response time, lower threshold current and lower sensitivity to temperature variations.
The p- or n-type layers of various band-gaps are typically grown as epitaxial layers from the alloys of III-V compounds. One common compound, gallium arsenide (GaAs), readily yields high quality single crystals. However, it has a band-gap of 1.43 electron volts (eV) which corresponds to the infrared end of the light spectrum.
A wider band-gap material must be used to produce an LED with emission in the visible spectrum. For example, efficient red LEDs have been fabricated from aluminum gallium arsenide (AlGaAs) semiconductor material. AlGaAs is lattice-matched to GaAs. The band-gap energy of semiconductor material can be increased with substitution of aluminum atoms for gallium atoms. The greater the aluminum substitution in the resulting material, the higher is the band-gap. Aluminum is chosen to form the alloy because the varying concentration of aluminum does not substantially affect the lattice constant, and this property allows successive epitaxial layers of lattice-matched AlGaAs to be grown easily.
Typically, to minimize re-absorption, the band-gaps of all layers are chosen to be wider than that of the active layer. In this way, these layers appear transparent to the light emitted from the active layer. By the same consideration, the substrate on which the epitaxial layer is grown should ideally have a wider band-gap. However, it is not possible to obtain AlGaAs in wafer form, and instead, the lattice-matched, visible light absorbing GaAs is commonly used as a substrate.
The AlGaAs-GaAs system can at best provide red LEDs and lasers. To obtain even shorter wavelength LEDs and lasers, such as in the orange-red or yellow part of the light spectrum, it is necessary to provide semiconductor materials with still higher band-gap energies which are capable of epitaxial growth to form the various junctions. To this end, two classes of semiconductor alloy systems have been proposed: one is non-lattice-matched and the other is lattice-matched.
In a paper by Osbourn, Biefeld and Gourley, published in Applied Physics Letters, Vol. 41, No. 2, July 1982, pp.172-174, there is disclosed a GaAs.sub.x P.sub.1-x -GaP system. This system is not lattice-matched, but the authors showed that layers can be grown with high crystalline quality if they are sufficiently thin strained-layer superlattices (SLSs) and that the composition of the layers is graded. In these structures, the lattice mismatch between layers is totally accommodated by strain in the layers, so that no misfit defects are generated at the interfaces. The authors fabricated an opto-electronic device which was shown by photoluminescence studies to have an emission at a wavelength of 611 nm (corresponding to a band-gap of 2.03 eV) at a temperature of 78K.
Since lattice matching is not required, SLSs can be grown from a wide variety of alloy systems and are consequently more flexible. Examples of other lattice-mismatched materials include GaAs-GaAsP. However, non lattice-matched systems cannot be grown as readily as lattice-matched systems, and their growth processes are generally more complex.
In the case of lattice-matched systems, it has been known that the AlGaInP-GaInP system can serve as the basis for growing higher band-gap devices. The substitution of aluminum for gallium in Ga.sub.y In.sub.1-y P has made possible the fabrication of high band-gap (Al.sub.x Ga.sub.1-x) .sub.y In.sub.1-y P-(Al.sub.z Ga.sub.1-z).sub.y In.sub.1-y P heterojunctions and quantum well heterostructures. Of these, the most important case is that of the (Al.sub.x Ga.sub.1-x).sub.y In.sub.0.5 P alloy (y approximately equals 0.5), which (similar to Ga.sub.0.5 In.sub.0.5 P) is lattice-matched to GaAs and yields shorter wavelength lasers than the Al.sub.x Ga.sub.1-x As system. It has a large direct band-gap up to 2.26 eV (549 nm), with potential for producing an emission wavelength in the range 555 nm to 670 nm at room temperature.
However, prior works with this system where active devices composed of (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P were formed directly on GaAs substrates have yielded devices with only moderately high band-gaps. Ishikawa et al, in Applied Physics Letters, no. 48, vol. 3, January 1986, pp. 207-208, obtained continuous (cw) room-temperature (300K) laser diode operation at the red wavelength of 679 nm. Among the shortest wavelength devices produced thus far is one disclosed by Kawata et al, published in Electronics Letters, no. 24, vol. 23, November, 1987, pp. 1327-1328. They reported a (cw) room-temperature laser at 640 nm using an (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P (x=0.15) active region in a double heterostructure configuration. As for pulsed, room-temperature operation, the shortest wavelength device is disclosed by Ikeda et al in Japanese Journal of Applied Physics, vol. 26, 1987, pp. 101-105. They reported a pulsed, room-temperature laser at 636 nm using an (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P (x=0.2) active region in a double heterostructure configuration.
The prior devices fall far short of realizing the full band-gap potential of the lattice-matched system described above. It is desirable to produce even shorter wavelength emitters by successfully growing possible higher band-gap devices under the lattice-matched system.